Traveling wave tube amplifiers (TWTA) are used to amplify and convert a low power input signal into a high power output signal and have been used for decades in a variety of applications. Modern satellite communications systems use TWTAs having high RF power output, high efficiency, minimum DC power consumption, and good linearity for low distortion. The design of TWTAs is usually based on a compromise among these requirements. A TWTA includes a traveling wave tube (TWT) in which electrons travel in an electron beam the physical length of the tube. Amplification occurs as a result of interaction between the electron beam and the electromagnetic fields produced by an input signal. An input signal modifies the velocity of the electrons so that the electron beam is modulated by an input signal. A cathode bias voltage, that is, an electron acceleration potential along the length of the tube, accelerates the electrons to the velocity required for the beam to interact with the electromagnetic fields produced by an input signal, thereby amplifying that input signal. The velocity of the electrons entering an input helix of the TWT is a function of the accelerating potential between the cathode and the helix. The electrical length of the tube depends upon the electron velocity, and therefore, the effective length of the TWT will be a function of the accelerating potential. Unwanted variations of the accelerating potential cause the tube to produce undesirable phase modulations of the output signal. Even though the accelerating potential is kept constant, high output signal levels can still cause the electrons in the beam to slow down. This decreased beam velocity causes the effective electrical length of the tube to increase, and thereby generates undesirable phase variations.
The radio frequency (RF) performance of the TWTA is primarily affected by two types of distortions, namely amplitude modulation to amplitude modulation (AM to AM) distortion (AM to AM conversion), and amplitude modulation to phase modulation (AM to PM) distortion (AM to PM conversion). Both types of distortion contribute to the generation of undesirable signal components including distortion products that accompany the desired high power output signal of the TWTA. These products are reflected in two conventional measurements, including third order intermodulation measurements and noise power ratio measurements. AM to PM conversion distortion occurs when variations in the instantaneous power of the input signal causes variations in the effective electrical length of the TWT. This AM to PM conversion distortion is small for low input power levels, but becomes more pronounced as the output power approaches saturation. Instantaneous variation of the input signal causes variation of the mean velocity of the electrons moving through the tube. Some of the kinetic energy of the electrons in the electron beam is ultimately converted to output signal power. As a result, the variation of the electron velocity causes a variation in the effective electrical length of the TWT, and therefore causes phase variations in the output signal. At higher output levels, a relatively large amount of energy is removed from the beam, which causes the mean electron velocity to be reduced. This causes the signal to propagate more slowly through the tube, and the tube becomes effectively electrically longer. Due to these interactions, the output phase is a nonlinear function of the instantaneous input power level.
AM to AM conversion distortion resulting in gain compression is caused by several factors that limit the amount of energy that is transferred from the modulated electron beam to the output helix. Included among the causes are complex changes in the electron beam density, and a reduction in the average velocity of the electrons in the beam. As a result, the slope of the amplitude transfer function decreases toward zero as the saturated power level is approached. Conventional linearizers contain circuitry for linearizing both phase and amplitude transfer functions. The conventional linearizer operates at the carrier frequency adding complexity and cost. Also, in the event that the linearizer fails, the entire TWTA will fail because the linearizer is in series with the input of the TWT. A failed linearizer would significantly attenuate the input signal. The traditional method of obtaining the required linearity as to both amplitude and phase has been to predistort the input signal or to operate the TWTA in a backoff condition, or both, resulting in a sacrifice in both power output and efficiency. Recently, linearizers have been employed to permit operation with reduced backoff while maintaining moderately high power output and efficiency. However, the linearizer increases the size, weight and complexity of the TWTA. Furthermore, the linearizers are matched to each TWT, preventing a general solution to TWT linearization. These and other disadvantages are solved or reduced using the invention.